我继承了一个长期工作的 postfix，它带有 courier imap 盒，运行普通的 smtp。它似乎使用 fail2ban 进行某种防火墙保护（从未使用过 fail2ban），根据 /var/log/messages，我们的盒子上收到大量尝试，这可能是正常的。
邮件机器上的 htop 看起来不错，除了内存使用量可能为 2.53（总内存使用量为 6 左右）。似乎有点高。无论从客户端单击电子邮件上的发送，电子邮件都需要几分钟才能最终“发送”。许多用户也退出了他们的客户端（我在 Android 上的 Outlook 应用程序上退出了）。我完全从手机上删除了该帐户并尝试添加它，但它无法连接到外发邮件服务器，尽管我知道密码是正确的。
（我可以在 /var/log/maillog 中看到这样的条目）
2020-06-22 09:45:45.459 xmail postfix/smtpd[2592]: 从 c-98-230-220-31.hsd1.nm.comcast.net[98.230.224.38] 连接
2020-06-22 09：48：39.527 xmail imapd：连接，ip=[::ffff：98.230.224.38]
2020-06-22 09：48：40.006 xmail imapd：登录，[电子邮件保护]，IP=[::ffff:98.230.224.38]，端口=[50411]，协议=IMAP
2020-06-22 09：48：55.932 xmail postfix/smtpd[2592]：从 c-98-230-220-31.hsd1.nm.comcast.net[98.230.224.38] 验证后丢失连接
...是我尝试连接手机时看到的。（在移动端，它只是失败并显示超时）
我不知道从哪里开始寻找速度变慢的原因，以及某些客户端无法连接或断开连接且无法重新连接的原因。
对我来说，这一切感觉像是某个地方（在服务器或客户端上）的认证问题，但不确定在哪里查找或如何检查该理论。我们确实有一个 cronjob，每周执行一次 cert-bot 自动更新，然后将目录更改为 /etc/letsencrypt/live/xmail.... 并将一些 privkey.pem、cert.pem 和 fullchain.pem 全部复制到 courrier.pem 中。然后它说 courier-imap-ssl restart 和 pop3d-ssl restart
我查看了日志，它没有告诉我任何错误，但邮件日志中确实说了很多类似的上述内容（关于连接被拒绝或断开等）
使用 Squirrel Mail 时，连接或发送邮件没有任何问题，运行非常顺畅。
所有这些问题似乎都是在大约一周到一个半星期前出现的，而且看来事情多年来一直进展顺利。
centos box courier imap postfix/smtp
当我尝试从我的客户发送电子邮件时，我的收件箱中也收到了一条新消息：
您的邮件未能送达部分或全部预期收件人。
Subject: postfix issue
Sent: 6/22/2020 1:31 PM
无法联系到以下收件人：
'[email protected]' on 6/22/2020 1:35 PM
Server error: '451 4.3.0 <[email protected]>: Temporary lookup failure'
这可能是我们的 DNS 服务器的 DNS 问题吗？
Shane ps 我刚刚从我的 WSL 安装的 Ubuntu 或 Mutt 中进行了简单的设置。配置它我思考正确，现在点击发送总是说：
无法连接到 mysmtp.blah.com（资源暂时不可用）。
所以也许可以解释为什么像 thunderbird 等客户端需要很长时间才能发送电子邮件？但不知道是什么会突然导致这种缓慢（重新启动了 VM imap/postfix 运行几次）
/var/log/maillog 中的一些日志条目 | grep 错误
2020-06-23 06:47:26.253 xmail amavis[7427]: (07427-01-7) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:47:41.895 xmail amavis[7111]: (07111-02-3) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:47:57.085 xmail amavis[7427]: (07427-01-8) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:48:11.329 xmail amavis[7111]: (07111-02-4) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:48:27.362 xmail amavis[7427]: (07427-01-9) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:48:48.961 xmail amavis[7111]: (07111-02-5) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:48:56.723 xmail amavis[7427]: (07427-01-10) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:49:21.196 xmail amavis[7111]: (07111-02-6) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:49:29.393 xmail amavis[7427]: (07427-02) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:49:51.207 xmail amavis[7111]: (07111-02-7) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:50:00.136 xmail amavis[7427]: (07427-02-2) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:50:29.001 xmail amavis[7111]: (07111-02-8) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:50:31.521 xmail amavis[7427]: (07427-02-3) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
2020-06-23 06:51:05.472 xmail amavis[7111]: (07111-02-9) (!)ClamAV-clamd av-scanner FAILED: run_av error: Too many retries to talk to /var/run/clamd.amavisd/clamd.sock (All attempts (1) failed connecting to /var/run/clamd.amavisd/clamd.sock) at (eval 132) line 659.\n
还有 /var/log/maillog 的尾部（Squirrel Mail 现在也需要一段时间才能发出消息。
2020-06-23 07:04:35.879 xmail imapd: LOGIN FAILED, [email protected], ip=[::ffff:127.0.0.1]
2020-06-23 07:04:36.690 xmail imapd: LOGIN FAILED, [email protected], ip=[::ffff:127.0.0.1]
2020-06-23 07:04:36.977 xmail imapd: Disconnected, ip=[::ffff:127.0.0.1], time=7
2020-06-23 07:04:36.988 xmail postfix/smtpd[5865]: warning: unknown[46.38.148.2]: SASL LOGIN authentication failed: authentication failure
2020-06-23 07:04:36.989 xmail postfix/smtpd[5865]: disconnect from unknown[46.38.148.2]
2020-06-23 07:04:36.990 xmail postfix/smtpd[5865]: connect from unknown[46.38.148.10]
2020-06-23 07:04:36.990 xmail postfix/smtpd[5865]: disconnect from unknown[46.38.148.10]
2020-06-23 07:04:36.990 xmail postfix/smtpd[5865]: connect from unknown[46.38.145.6]
2020-06-23 07:04:36.995 xmail imapd: Connection, ip=[::ffff:127.0.0.1]
顺便说一下，这个 @nmsu.edu 我们已经看到了很多尝试，这几乎就像有人拿一本名字字典并连接 @nmsu.edu 并看看哪些会成功？我们在这个服务器上以某种方式运行了 fail2ban（我通过这个不断了解更多有关它的信息）